Retroreflective sheetings have the ability to redirect incident light towards its originating source. This unique ability has led to the wide-spread use of retroreflective sheetings on a variety of articles. Very often the retroreflective sheetings are used on flat inflexible articles, for example, road signs and barricades; however, situations frequently arise which require the sheetings to be used on irregular or flexible surfaces. For example, a retroreflective sheeting may be adhered to the side of a truck trailer, which requires the sheeting to pass over corrugations and protruding rivets, or the sheeting may be adhered to a flexible substrate such as a road worker's safety vest. In situations where the underlying surface is irregular or flexible, the retroreflective sheeting desirably possesses good conformability and flexibility but not at the expense of sacrificing retroreflective performance.
There are essentially two types of retroreflective sheeting: beaded sheeting and cube-corner sheeting. Beaded sheeting employs a multitude of glass or ceramic microspheres to retroreflect incident light. The microspheres are separate from each other and therefore do not severely impose on the sheeting's ability to be flexed. Cube-corner sheeting, on the other hand, typically employs a multitude of rigid, interconnected, cube-corner elements to retroreflect incident light. Due in part to their interconnected nature, the shape of the cube-corner elements can become distorted during flexing, resulting in a loss of retroreflectivity. The construction of cube-corner sheeting, therefore, places limits on the degree to which the sheeting can be conformed or flexed and still maintain satisfactory retroreflectivity. In the attempt to expand or remove these limits, investigators have taken many different approaches to produce a cube-corner sheeting which demonstrates good retroreflectivity after being flexed or conformed. Examples of these different approaches have been disclosed in U.S. Pat. Nos. 3,684,348, 3,924,929, 3,992,080, 4,555,161, 4,576,850, 4,668,558, 4,582,885, 5,177,304, 5,189,553 and U.K. Patent GB 2,245,219 A.
In U.S. Pat. No. 3,684,348 a retroreflective sheeting is disclosed that has a multitude of cube-corner formations projecting from a body portion. The cube-corner formations and the body portion are separately formed from essentially transparent synthetic plastic resin and are bonded together as a composite structure. To facilitate mounting on and shaping to surfaces of various configurations, it is disclosed that the body portion can be flexible.
U.S. Pat. No. 3,924,929 discloses a cube-corner retroreflective sheeting that contains a multiplicity of trihedral prismatic retroreflective units separated into cells by interconnected sepia. A multiplicity of retroreflective sheeting units may be bonded to a flexible backing such as a polyester or polyvinylchloride web. The retroreflective sheeting units are positioned on the backing in a manner that allows the flexible article to be rolled or folded in either direction along two adjacent edges.
U.S. Pat. No. 3,992,080 discloses a retroreflective sheeting that provides good retroreflection when stretched. The sheeting comprises a first strip of transparent flexible synthetic resin having a multiplicity of minute cube-corner formations on one surface thereof. The cube-corner formations are bonded to a second strip of flexible backing material of lesser length than the first strip when in a relaxed condition. The first strip is bonded to the second strip with the cube-corner formations disposed adjacent to the second strip. This composite retroreflective sheet material is puckered in the relaxed state and is stretchable on a support surface with the elimination of the puckered condition. It is disclosed that this construction allows the sheeting to be stretched while avoiding distortion of the cube-corner formations.
U.S. Pat. No. 4,555,161 discloses a retroreflective sheeting that provides a high degree of flexibility to permit the sheeting to be tailored to a wide range of applications. The retroreflective sheeting comprises a base sheet that includes a flexible synthetic plastic sheet material; a cover sheet that includes a coextensive length of flexible transparent synthetic plastic; and a multiplicity of retroreflective film pieces disposed between the base sheet and the cover sheet. The film pieces are arranged as an array in a predetermined pattern, and the sheets are bonded to one another in areas between and about the film pieces to provide a multiplicity of discrete cells in which the film pieces are seated. A portion of each of the film pieces defines a multiplicity of minor cube-corner formations that provide the retroreflective properties to the sheeting.
U.S. Pat. Nos. 4,576,850, 4,582,885, and 4,668,558 disclose a retroreflective cube-corner sheeting that possesses good flexibility and dimensional stability. The retroreflective sheeting is made from a cross-linked polymer composed of (1) a plurality of hard segments of mono- or polyvalent moieties containing one or more carbocyclic and/or heterocyclic groups and (2) a plurality of soft segments of mono- or polyvalent moieties. The moieties of the hard segments have a major transition temperature above 250.degree. K., and the moieties of the soft segments have a glass transition temperature below 250.degree. K. and have an average molecular weight of about 500 to 5,000.
U.S. Pat. No. 5,117,304 discloses a flexible retroreflective sheeting based upon an optically clear, aliphatic polyurethane polymer. The retroreflective sheeting comprises a land and an array of elements on the land. The flexibility is imparted to the sheeting by use of an aliphatic polyurethane polymer in the retroreflective elements which has a plurality of hard chain segments of the formula --C(O)N(H)--C.sub.6 H.sub.10 --N(H)C(O)--.
U.S. Pat. No. 5,189,553 discloses retroreflective cube-corner sheeting that is suitable for bending applications. The sheet has an outer surface that is designed for tension during sheet bending and an inner surface design for compression during sheet bending. A sheet neutral bending access region is located relative to the outer and inner surfaces so that a neutral bending access exists which is substantially free of stress and deformation during sheet bending. The sheet also has a plurality of reflective cube-corner elements with surfaces purposefully located proximate to the neutral bending access. It is disclosed that this sheeting provides enhanced retroreflective performance for a given radius of curvature.
U.K. Patent Application GB 2,245,219 A discloses a flexible retroreflective sheet material comprising relatively flexible body member of a transparent synthetic resin. The flexible body member has first and second faces, where the first face is planar, and the second face has closely-spaced retroreflective microprisms located thereover. The body member has a thickness from the first face to the base of the microspheres of 5.08 to 25.4 micrometers. The microprisms have a height of 25.4 to 254 micrometers. An adhesive coating is disposed on the second face over some of the microprisms, and a flexible backing member extends over the second face and is bonded to the microprisms by the adhesive coating.
Although the above-discussed patents disclose a variety of different constructions for providing flexible, retroreflective, cube-corner sheeting, some of the disclosed constructions are relatively complicated in construction. Others, while being no more complicated than a typical retroreflective sheeting, use relatively expensive polymeric materials or provide limited retroreflectance when highly flexed or conformed, or simply fail to fully teach how good retroreflectivity is achieved after the sheeting has been flexed or conformed.